[GHC] #13099: recompilation can fail to recheck type family instance consistency

[GHC]

#13099: recompilation can fail to recheck type family instance consistency
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        Reporter:  rwbarton          |                Owner:  rwbarton
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 I think the problem is that (1) doesn't go far enough: it incorporates
 into the hash the list of orphan modules that we transitively hash, but
 NOT the actual LHSes of the type family instances. So #12723 handles if we
 rename a module, but not if we change an LHS (which can also invalidate a
 consistency check.)

 As always with these sorts of bugs, a failing test case helps a lot!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13099#comment:4>
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